[The_Reader_David]

Actually, most of the non-green areas in the map would have been the same 500 or 1000 years ago: prairie, desert, and mountain tundra.

[Deaf Smith]

"non-green areas in the map would have been the same 500 or 1000 years ago"

Well, if you don't count the mesquites that the Spaniards gave Texas. The Spaniards brought over mesquite beans to feed their horses while on their walk about.